Need help please.. What would make my rpms stick when engaging the clutch in my 99 C3500 Sierra? Also, while driving at average speeds and when letting off the gas, the deacceleration/decompression is hesitant. The throttle body was cleaned and the spring replaced yet still having this problem and im affraid it's going to burn out my clutch. Thanks!!

 

The idle air control valve on the side of the throttle body could be sticking or not working at all causing high idle. It's cylindrical and held in with torx screws. If it's leaking that way the air is metered and the engine would be strongly pulling. If it's rough then could be a vacuum leak.
If this is a smallblock when were the intake manifold gaskets last done and is it losing coolant? Vacuum leaks can also come from the PCV hose or the brake booster, but that truck should have hydroboosted brakes being a 3500.

I should also mention - do not attempt to adjust the throttle stop setscrew, that is calibrated at the time of TB manufacture and should not be changed as doing so could cause the blade to stick closed.